Squelch circuit



P 1951 R. F. SMELTZER 2,569,348

' SQUELCH CIRCUIT Filed May 28, 1946 RAYMOND F. SMELTZER Patented Sept. 25, 1951 SQUELCH CIRCUIT Raymond F. Smeltzer, Towsn,-Md., assignor to Bendix Aviation Corporation, Towson,,Md., a

. corporation of Delaware Application May 28, 1946, Serial No. 672,746

This invention relates to circuits for the quieting'of sensitive radio receivers in the absence of useful signals and more particularly to a circuit of this type for use in either frequency or amplitude modulated receivers.

A number of noise suppressing or squelch circuits have been developed for use in frequency modulation receiving equipment. Such circuits usually operate by blocking the audio amplifier of the receiver until a combined frequency and noise modulated signal is received, the magnitude of which exceeds a predetermined threshold level.

Arrangements of this nature have proved unsatisfactory because the receiver tends to be sensitive to noise in noisy locations, and weak but use- .ful frequency modulation signals often fail to opcrate the receiver in quiet locations. Since the squelch circuit is responsive to the combined signal and noise amplitude, it can be adjusted for satisfactory operation only in the rare location 4 where the noise level is constant. Even in such :a location most, of such circuits are defective in another respect, in that a border line signal is reproduced in distorted form, modulated by an exaggerated replica of the integrated amplitude fluctuations of the signal and noise.

It is, therefore, an object of this invention to provide a squelch circuit which is responsive only are realized by the provision of a squelch circuit in which the signal is fed to two detectors responsive to different forms of modulation. One of the detectors is responsive to the form of modulation employed in the desired signal to render the intelligence it conveys, the other being responsive only to noise and rendering an indication of the noise in a form suitable for the operation of a means for blocking a succeeding audio amplifier. Two or more rectifiers are employed Claims. (01. 250-) in cascade in the noise detector with delayed bias of which there is shown a schematic diagram of a portion of a frequency modulation receiver circuit embodying the invention.

Referring now more particularly to the drawing, there is illustrated a conventional frequency modulation detector III connected between the limiter II and the first audio tube I4 of a frequency modulation receiver. The tube I4 is a dual diode-triode, the audio amplifying portion of the tube comprising the anode 29, control grid 28 and cathode 30. A pair of diode anodes l3 and I5 are associated with the cathode 30. A condenser I2 provides a connection between the output circuit 21 of the limiter II and the diode anode-I3. The diode anode I5 is connected through acondenser 26 and a resistor I6 to the anod I3, and through a condenser 20 to ground. A resistor I1 is connected between the anode I3 and ground. The junction of the condensers 20 and 26 is connected to ground by a resistor 2| in series with a parallel circuit comprising a resistor 23 and the condenser 22. The junction of resistors 2| and 23 is connected to the control electrode 28 through v a resistor 24. A condenser 25 provides coupling means to the control electrode for signals issuing from the detector Ill. A resistor I8 Which is shunted by a condenser I9 provides a connection between the cathode 30 of the tube I4 and ground.

In operation, noise signals appearing as an amplitude modulated disturbance in the output of the limiter stage II are applied to the anode I3 of the tube I4 through the condenser I2." It is to be noted that no detection of these signals will take place until their amplitude exceeds the bias voltage caused by the triode space current flowing through the resistor I8. If the noise signals are above thisthreshold, amplitude detected signals will appear across the diode load resistor These detected signals are applied to the anode I5 through the resistor I6 and the blocking condenser 26. It will be seen that th signal applied to the anode I5 must also be greater than the voltage across the resistor I8 for current flow. If this threshold level is exceeded the signals detect'ed at theanode I3 will be rectified and integrated by the circuit comprising the anode I5, the integrating network comprising the resistor 2I and the condenser 22 and the further integrating network comprising the resistor I6 and the condenser 20. Theintegrated signal is applied to the control electrode of the tube I4 through the decoupling resistor 24 where it appears as a bias which tends to decrease the flow of anode current through the triode I4. This decrease in anode current causes a decrease of voltage across the resistor I8 which lowers the operating threshold of the system. It will be manifest that the above described arrangement is highly regenerative and that the triode I4 is substantially in a non-conducting state irrespective of the amplitude of the applied noise signal provided it exceeds the threshold value. Operation of the tube l4 as described above 'requirest-hat aminimum bias greater than one half cut-off :val-ue beused.

The use of the two RC combinations l6, 2!] and 2|, 22, and the connection of the final load resistor 23 across their output, causes the tube M to unblock at a rate commensurate with the rate at which it blocks, thus holding the cross-over time between stages of equilibrium 'to a very small value. If it is not considered necessary to restrict cross-over time to such values, either of the RC combinations i6, 29 or 21, 22 may be dis pensed with and the load resistor 23 may be connected to ground from any portion of th circuit between the anode I and the control grid-28 of the tube I4.

Although a dual diode-triode type. of tube is illustrated, it will be manifest that separatetubes may be used if desired. This may be necessary if-anexcessive degree of coupling is found to be caused by the construction of the combination tube.

Since many circuit variations falling within the scope of-the invention as defined by the.ap-

pended claims will'suggest themselves to those skilled in the art, th invention is not to be. considered as limited to the embodiments disclosed v to theenergy of a desired signal; an electricdischargedevice comprising an anode, a cathode and a control electrode; meansimpressing the output of said detector uponflsaid control electrode; a resistance connected ibetweensaid cathode and ground; -a condenser shunting .said resistance; a pair of auxiliary anodes associated with said cathode and constitutingwith said cathode a pair :ofdiodes; meansconnecting said diodes in cascade; means impressing the energy applied to said detector upon the first of said cascaded diodes; means integrating theoutput of said diodes and means impressing the integrated output upon said control grid.

2. In a receiver of radiant energy signals: a squelch circuit comprising a detector responsive to the energy of a desired signal; an electric discharge device comprising an anode, a cathode and a control electrode; means impressing the output of said detector upon said control elec-- trode; a resistance connected between said cathode and ground; a condenser shunting said resistance; a'pair of rectifiers connected in cascade,

each of said rectifiers having an anode and a' cathode, the cathodes of said rectifiers and said electric discharge device all being directly connected to a common point, means for connecting theuanodes of said rectifiers to ground, whereby a bias will be impressed upon said rectifiers upon the occurrence of space current flow through said discharge device; means impressing the energy applied to said detector upon the first of said cascaded rectifiers; means integrating the output of said rectifiers and means impressing the integrated output upon said control grid.

3. In a receiver of ,radiant energy signals; a squelch circuit comprising a detector responsive to the energy of a desired signal; an electric discharge device comprising an anode, a cathode and a control electrode; means impressing the output of said detector upon the said control electrode; a pair of rectifiers connected in cascade; means impressing on said rectifiers a bias responsiveto space current flow through said device; means impressing the energy applied to said detector upon the first of said cascaded rectifiers; means integrating the output of said rectifiers and means impressing the integrated output upon said control grid.

4. In 'a receiver of radiant energy signals: squelch circuit comprising .a detectorresponsive to the energy of a desired signal; an'audiocainplifier; means impressing the output of said detector upon the inputof said amplifier; a :pair of rectifiers connected in cascade; means biasing said rectifiers inresponse to space current'iiow through said amplifier; means impressing the energy applied to said detector on the first of said cascaded rectifiers means integrating the output 'of said rectifiers and means impressing the integrated output upon the input of said amplifier.

5. An electrical signal transmissioncircuit'ineluding a space disch'argeipath' and 'havingxtwo conditions of stable equilibrium, in the firstiof which current flows in said path and'in thesecond-of which said path is non-conductingccomprising: 'a"controlzelectrode'for regulating current flow insaid path; .an impedance-insaidpath across which is derived by said current now a biasing voltage for said control electrodeya pai-r of rectifiers biased by said derived voltage;..means applying signal voltage to :a first offs'aid' rectifiers; means applying the alternating 'portionbffthe output of saidv first rectifier to .t'hesecondof said rectifiers; and means integrating the output of said second :rectifier' and :applying'said integrated output to said control electrode.

RAYMOND F. 'SMELTZER.

REFERENCES CITED The following references are of record in the file 'of this patent: 

